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American Mi ner al o gist, Volume72, pages 863-878, 1987

Evolutionof magmaticAFM assemblagesin granitoid rocks: The * melt : reaction in the Libertv Hill pluton, South Carolina

J. Ar,nxaNDER SPEER* OrogenicStudies Laboratory, Department of GeologicalSciences, Virginia Polytechnic Institute and StateUniversity, Blacksburg,Virginia 24061, U.S.A.

AssrRAcr The Liberty Hill pluton comprisesa central, coarse-grainededenite + biotite granitoid faciesgrading to a biotite, + biotite, or * biotite granitoid marginal facies.The replacementof by biotite, mineral compositions, and "liquid" vari- ation diagrams for the Liberty Hill help determine the AFM mineral-crystallization re- actions in the Liberty Hill pluton. These reactions evolved from liquid : amphibole to liquid : biotite with an intervening liquid + amphibole : biotite reaction resulting from either (l) movement along the amphibole-biotite liquidus boundary from the even, liq- uid: amphibole * biotite, to the odd, liquid + amphibole : biotite, crystallization reaction with decreasingtemperatures or (2) movement of the liquid composition from the amphibole field into the biotite field. The liquid + amphibole : biotite reaction involved additional phases;the full reaction may have taken the form liquid I + amphi- bole*:biotite+qtrarlz+anorthite+K-+titanite+magnetite+ liquid 2, where the liquid is the interstitial melt. The extended crystallization history to lower temperaturesand higher water activities at the margins of the pluton allowed the reaction to exhaust the amphibole before the liquid; the subsequentliquid : biotite crys- tallization reaction produced the biotite granitoid facies. Locally, muscovite crystallized accordingto the reaction liquid : muscovite + biotite, producing the muscovite + biotite facies. The pluton is intruded by a finer-grained biotite granitoid believed to be derived from interstitial melt of the coarser-grainedgranitoids. This derivation explains why bio- tite is the only AFM mineral in the finer-grained granitoids and why from all Liberty Hill granitoids are generally similar in composition. The averageKo value l(XBJe/XB;)/(W;/Xf"b')lis 1.04 for coexisting Hbl and Bt. This K" is higher than the 0.69-0.93 normally observed in igneousrocks, but equal to values for specificinstances of biotite replacing amphibole in the Peruvian Coastal batholith. Com- bined with the results obtained from the contact aureole, the central granitoids preserve conditions of 725"C,P,oor E 4.5 kbar, Po",oE 0.5 P,"o,,andfo, - 10-15,slightly more oxidizing than the NNO buffer. Marginal granitoids yield conditions of 647"C, Po,,or P,.^,, andfo, I 10-16,suggesting a more prolonged reaction history.

INrnonucrroN ciesoverlap thoseofthe centralgranitoids, arguing against Many of the late Paleozoic granitoid plutons of the significant differentiation (Speeret al., unpub. ms.). The southernAppalachians are composite bodies with coarse- marginal facies do show minor wall rock- inter- grained amphibole + biotite and biotite varietal facies action that affectedsome trace-elementand oxygen-iso- (Speeret al., 1980). From field relations, thesefacies ap- tope compositions(Wenner and Speer,1986). An alter- pear closelyrelated, and multiple intrusion or differentia- native explanation for the mineralogical relation between tion comesto mind as an explanation for the relationship. the central and marginal facies is suggestedby the wide- In the Liberty Hill pluton, the coarse-grainedcentral am- spreadtexture of biotite replacing amphibole. phibole + biotite granitoids grade imperceptibly into Understanding the mineral assemblagesand composi- marginal biotite granitoids, seeminglyruling out multiple tions in igneousrocks requires linking them to the evolv- intrusions. The compositions of the marginal biotite fa- ing conditions and compositions of the interstitial melt during crystallization. This is a well-developed area of gabbroic * Present address:Department of Marine, Earth and Atmo- investigation for rocks, but much less so for spheric Sciences,Box 8208, North Carolina State University, granitoid rocks. Recently, Abbott and Clarke (1979) and Raleigh,North Carolina27695-8208, U.S.A. Abbott (1981, 1985) have given the sequenceofAFM 0003404x/87/09 I 0-0863$02.00 863 864 SPEER:EVOLUTION OF MAGMATIC AFM ASSEMBLAGES IN GRANITOID ROCKS crystallization reactionsin granitoids in more detail than given in Tables 1 to 3. Mineral formulas and componentswere Bowen's(1928) reaction series. These have beenused in calculatedusing the computer program suPERrncer-ofRucklidge program stoichiometric interpreting the crystallization reactions in strongly per- (1971). The also calculatedan assumed amount of water and added it to the oxide weight percentages aluminous granitoids, such as the -bearing for the hydrous . Mineral abbreviations in the text, ta- pluton, South Carolina (Speer,198 1b). Their CloudsCreek bles, and figuresare those recommendedby The American Min- use is extended here to the metaluminous Liberty Hill eralogist(ketz, 1983). pluton, South Carolina, in order to understand the evo- lution of amphibole + biotite, biotite, and muscovite + Amphibole biotite granitoids. Amphibole occursas euhedralto subhedralprismatic crystals up to 0.5 cm long. They are pleochroic dark yellow green to Gnolocrc sETTTNG moderate blue green and are locally twinned on (100). Micro- The Liberty Hill pluton is roughly elliptical, covering probe analyses(App. 1, selectedanalyses in Table l) show that 360 km'zin Kershaw, l,ancaster, and Fairfield Counties, the in the central granitoids are ferro-edenitesand north-central South Carolina. The Liberty Hill pluton ferro-edenitic (Fig.2a), hereafterreferred to as fer- comprises severaltexturally and mineralogically distinct ro-edenites.These ferro-edeniteshave a narrow compositional Fe/(Fe + Mg) near 0.6 and a Si content in the facies(Fig. 1). The predominant faciesis a coarse-grained range with an half-unit-cell formula of 6.56 to 6.85 atoms per 24 anions. The ferro- + biotite granitoid occupying the center of F and Cl contents rangebetween 0.08 and 0.22 molo/oF/(OH + pluton. pargasite the Coarse-grainedbiotite, ferroan + F + Cl) and between0.005 and 0.03 mo10/oCV(OH + F + CD, biotite, and muscovite * biotite granitoids occur along respectively.Amphiboles in granitoids of the northeast corner the northern and easternborders ofthe pluton and have of the pluton have thin, discontinuousaluminous rims of ferroan gradational contactswith the central ferro-edenite * bio- pargasite(Fig. 2B). Thesealuminous rims are lessthan 0.01 mm tite granitoids. This reversezoning of the mineral facies thick and are only locally visible, where there is a color difer- is believed to result from accumulation of magma with a ence. According to graphical representationsof Doolan et al. greater percentageof evolved, interstitial melt near the (1978)and Thompson(1981), the substitutionin the aluminous margin as a result of the pluton's emplacementas well as rims is primarily a [AlrMg 'Si-'] + edenite to form pargasite. a minor amount of contamination from the wall rocks [NaAlSi-,] substitution Many amphiboles are intergrown with biotite, but generally (Wenner and Speer, 1986). There are severalyounger fa- the order of crystallization is uncertain. Where textures are un- pluton cies in the Liberty Hill that intrude the coarse- ambiguous,the amphiboles are partially (Fig. 3a) or completely grained granitoids as dikes or plugs up to I km2 in area. (Fig. 3b) replaced by biotite. In what is interpreted as a later The most prominent are fine- to medium-grained biotite reaction texture, amphiboles can be replacedby chlorite + epi- granitoids in the west-centralpart ofthe pluton (Fig. 1), dote + .Scattered throughout the pluton are amphi- which are believed to have differentiated from the coarse- boles with clinopyroxene inclusions (Fig. 3c). The amphibole grained facies. adjacent to these is pale green actinolite that is dis- The contact aureole of the pluton was described by continuously zoned to ferro-edenite at the rim, comparable in pluton (Fig. Speer(198 I a). The pelitic assemblagein the country rock, composition to amphiboles elsewherein the 2b). muscovite + chlorite + albite * + + Biotite magnetite + ilmenite, changesin responseto prograde Greenish-blackbiotite is a ubiquitous phasein the Lib- pluton. on approachingthe The following erty Hill pluton, forming up to 5 modal percent of the rock. isograds are encountered:porphyroblastic magnetite in, Biotite occursinterstitially to feldsparsand quartz, replacingam- epidote + albite out, biotite + cordierite in, chlorite out, phibole, and as inclusions in leucocratic minerals. Individual K-feldspar in, and muscovite or magnetite out. Several flakesare up to 5 mm across.Biotite is pleochroic grayish orange additional metamorphic minerals appearin the xenoliths: to greenishblack in thin section. , orthopyroxene, andalusite, sillimanite, and fi- Microprobe analyses(App. l, selectedanalyses in Table 2) brolite. It was concluded that the pluton was emplaced show that biotite compositions from the granitoids fall approx- phlogopite at a depth correspondingto 4.5-kbar pressure.The mag- imately midway between and annite with an inter- The range of molar Fel(Fe + Mg) is small ma was relatively dry, and the xenoliths yield estimated mediate Al content. with an averageof approximately 0.59. The amount of tetrahe- temperatures of about 725C for the central granitoids dral Al is systematicallyrelated to the coexisting minerals (Fig. granitoids. and 650'C for the maryinal 4). Biotites coexistingwith ferro-edenitehave tetrahedralAl con- MrNnnar,ocv tents between 2.27 and 2.35 atoms per 24 anions. The tetrahe- dral Al contentsof biotites coexistingwith the pargasite-rimmed Compositionsof the mineralsin polishedthin sectionswere hornblendesare decidedly more aluminous, in the range2.29 to determinedwith a nine-spectrometer,automated,qtr-snr're elec- 2.75 atoms per 24 anions. Biotites in the fine-grainedand coarse- tron microprobeusing silicates and oxides as standardsand the grained biotite granitoids and enclaveshave tetrahedralAI con- analyticalscheme qar-r described by Solbergand Speer(1982). tents that overlap the biotites coexistingwith ferro-edenite.The Theseare reportedin Appendix l,' and selectedanalyses are biotites of the finer-grainedrocks can be more iron rich, with an Fe/(Fe + Mg) : 0.64; in biotites from the aplites, Fe/ rTo obtain a copy of Appendix 1, order Document AM-87- average 355 from the BusinessOffice, Mineralogical Societyof America, (Fe + Mg) is as high as 0.75. The biotites in coarse-grained 1625I Street,N.W., Suite414, Washington,D.C. 20006,U.S.A. muscovite + biotite granitoids are aluminous and comparable Pleaseremit $5.00 in advance for the microfiche. in composition to biotites coexistingwith the pargasite-rimmed Fig. 1. Geologic and sample maps of the Liberty Hill pluton, South Carolina. 866 SPEER:EVOLUTION OF MAGMATIC AFM ASSEMBLAGES IN GRANITOID ROCKS

omphiboles(No * K)a> 0.5 F) is 0.06 to 0.20. Biotites in the coarse-gainedbiotite granitoids telrohedrolsilicon are more F rich with an averageF/(OH + F) of 0.17. The biotites in muscovite + biotite and fine-grained biotite granitoids are , ^8.O0 700 600 r.v r- lessF rich with averagevalues of F/(OH + D of 0.05 and 0.09, q edenilic respectively. o l! Clinopyroxene + .- ferroon porgosrtrc Pyroxenein the Liberty Hill pluton occursas very pale green, hor irregular or skeletalcores in amphiboles(Fig. 3c) and in the more o calcic .Microprobe analyses(App. l, selectedanal- *: ferro-edenilic yses the to be salite with an average ferro-edenite hornblende in Table 3) show 00 compositionWoou orEn' u,FsrorrRdn, or. tetrohedrolsilicon Coexisting AFM minerals Figure 5 summarizes rock and mineral compositional rela- tions from the Liberty Hill pluton. Rock compositions are more iron rich than the ferromagnesiansilicate assemblagebecause of o octinol ile L the presenceofiron oxides. + A notable aspectof Figure 5 is the differing distribution coef- o ^_ >(J5 ficients for coexisting amphiboles + biotite, evident by the slopes olr of tie lines, and differing Al contents. The distribution coefr- cient, defined as Ko : (X',;JX";)/(W;/XF9, depends on the overall mineral assemblageand bulk composition. In the en- 00 clavesand enclave-bearingrocks, assemblageswith clinopyrox- (No+KL.

TABLE1. Selectedamphibole compositions for the LibertyHill pluton,South Carolina

Amphibolesfrom the Zoned amphiboles central granitoids Cpx-Tr-Ed-Hbl Ed-Hbl - Prg s6-s3 s6-53 s6-86 s6-86 56-86 core nm core mid. rim 56-56 F6-24 K3-1303 44.20 43.06 sio, 52.75 45 38 43.94 42.41 40.13 43.59 42.30 ',1.46 Tio, 0.16 1.45 1.68 1.09 1.53 1.82 1.37 1.57 Al,03 2.05 7.69 7.56 9.28 13.O2 8.59 9.41 7.93 8.73 FeO 16.63 21.19 22.30 23.05 19.92 20.26 21.68 21.56 21.28 MnO 0.61 0.58 0.80 o.75 0.73 0.71 0.70 o.74 0.85 Mgo 13.85 9.39 8.17 7.84 7.18 8.73 7.92 8.35 8.20 CaO 11.85 11.40 10.58 10.79 10.16 10.69 11.33 11 .53 10.34 BaO 0.04 0.08 0.01 0.05 o.22 Naro 0.75 1.86 1.85 1.84 1.35 1.68 1.61 1.49 1.70 KrO 0.30 1.05 1.02 1.27 1.00 1.02 1.14 1.01 1.13 F 0.89 o.77 0.52 0.69 0.51 0.47 0.59 o.44 0.41 cl 0.02 0.22 0.12 0.07 0.15 Hro. 1.63 1.58 1.62 1.59 1.67 1.73 1.65 175 1.69 -O=F + Cl 0.38 0.37 o.29 0.31 0.21 0.20 0.25 0.19 0.21 't02.27 Total 101.15 99.90 100.41 96 99 99.09 99.45 100.38 99.01

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S)JOU CIOJINVUC NI SADV'ItrI^ISSSV WCV CIJVI{CYW CO NOIJN'IOAS :UAAdS 898 SPEER:EVOLUTION OF MAGMATIC AFM ASSEMBLAGES IN GRANITOID ROCKS 869 laete 2-Continued The reddish-grayilmenite grains contain networks and blebs of white-gray hematite and indicate exsolution of an original iI- menite-hematite solid solution. The ilmenites contain on aver- age12molo/opyrophanite, in the rangeof 6.4 to 18.4molo/o (App. Fine-grained granitoids Bt Aplite 1). Calculation of the temperatureand oxygenfugacity from mi- JO-O/ s6-99 56-101 K3-72 K3-130 s666 s677 croprobe point analysesof exsolved oxides in a ferro-edenite + granitoid(56-56) (1983) 36.45 35.46 36.78 37.49 36.53 3719 36.34 biotite by the methodof Stormer shows 2.19 3.55 2.54 2.47 2.63 3.72 3.14 subsolidusre-equilibration, below the range ofthe calibrations, 16.12 14.96 16.11 16.29 15.27 14.77 13.31 consistentwith the observedtextures. 24.39 23.78 21.47 24.04 25.09 25.97 24.82 In the coarse-grainedbiotite and muscovite + biotite ganit- 0.37 0.45 0.57 o.22 0.36 0.50 0.71 granitoids, 7.31 8.34 8.76 7.32 6.82 4.94 8.52 oids and in the fine-brained biotite the magnetitesdo 0.05 0.16 0.04 0 09 0.06 0.06 0.06 not contain exsolved phasesand only rarely does the ilmenite 0.25 0.93 0.23 0.07 0.07 have exsolved hematite. This indicates that the composition of 0 09 0.09 002 0.03 0.10 0.o7 0.13 the original igneous oxides in these granitoids must have been 1002 8.88 9.45 9.37 9.58 9.24 9.31 0.91 0 68 0.72 0.65 0.76 0.82 closer to the end-member compositions than were the oxides in 0.05 012 0.07 0.02 0.08 the ferro-edenite + biotite granitoids. 3 45 3.51 3 57 3.94 3.54 3.49 3.43 Two sulfideassemblages have been observedin the granitoids; 0 39 0 31 0.30 0.29 o.32 0.36 an inclusion assemblagein the magmatic minerals and a matrix 101.26 100.60 99.73 10126 100.64 100.48 100.38 assemblage.Pyrrhotite + chalcopyrite + pyrite is the inclusion

b ////////////// rocKonoryses -ol o omphibole N -o2 edenite/ hornblende (f z I -o3 cenlrolcoorse-groined gronitoids N )< -04 I o clinopy?oxene

o O I ol rO o , +' htnltla 1J _JU oo rocKonolyses

o -ol E amphibole

edeni le / hor nblende

morginol,coorse-groined groniioids ;! finer-groined biotitegroniloids -03 . omphibole+ biotiie O oplites tr biotile -o4 A biotite+ muscovite

o3 o4 o5 o6 o7 U5 o4 U) o6 o7 mol.MgO / (MgO+ FeO) Fig. 6. Summary AFM (A : Al,O3-I(rO-NarO-CaO, F : FeO, M : MeO) diagrams projected from quartz, alkali feldspar, plagioclase,and water for the rocks and minerals ofthe Liberty Hill pluton, South Carolina, differentiatedby occurrence:(a) enclaves and enclave-bearinggranitoid, (b) central, coarse-grainedferro-edenite + biotite granitoid, (c) maryinal, coarse-grainedbiotite, ferroan pargasite+ biotite, and muscovite + biotite granitoid, and (d) finer-grainedbiotite granitoid and aplite facies. 870 SPEER:EVOLUTION OF MAGMATIC AFM ASSEMBLAGES IN GRANITOID ROCKS assemblagein magnetiteand less commonly in silicatessuch as DrscussroN titanite and zircon. No sulfide inclusions have been observedin ilmenite. The matrix assemblageis pyrite * chalcopyrite and Conditions of crystallization coexistswith magnetite and ilmenite. From the contrasting oc- The probable conditions during crystallization of the currenceofthese two sulfide assemblages,it appearsthe pyrrho- granitoids can be deduced from contact tite + chalcopyrite + pyrite is the initial sulfide assemblagethat Liberty Hill xenoliths crystallizedwith magnetiteand the early silicates.Pyrite + chal- metamorphic assemblagesof the aureole and copyrite is the latter sulfide assemblagethat crystallizedin equi- and, to a lesserextent, from compositions of the igneous librium with both magnetite and ilmenite. minerals. Speer(198 la) found that the depth of emplace- Other accessoryminerals in the Liberty Hill pluton are allan- ment correspondsto a pressureof 4.5 kbar on the basis ite, apatite, bastnesite(?), cofrnite, fluorite, galena,molybdenite, of Fe-Mg partitioning in the assemblagecordierite + gat' monazite, pyrophanite, sphalerite, thorite, titanite, uraninite, and net * orthopyroxene + qvartz. This is comparableto the zircon. The molybdenite occurrencein the Liberty Hill pluton estimatedpressures of 3. I to 4.7 kbat calculatedfrom the (1978). has been discussedby Speer The U- and Th-bearing A1 content of the ferro-edenitesusing the geobarometer minerals have beendiscussed by Speeret al. (l 98 l). The apatites of Hammarstrom and Znn (1986). Water pressure was are fluor-apatites,with only minor OH substitution and lessthan half the total pressurein the central gran- 0.06 wto/oCl. Fluorite is a rare accessorymineral in the finer- suggestedto be meta- grained granitoids, occurring as disseminated anhedral grains. itoids on the basisofthe occurrenceofdehydration Allanite is the ubiquitous REE primary phase in all granitoids morphic reactions in xenoliths at temperatures much except for the coarse-grainedmuscovite + biotite granitoid where lower than they would occur under conditions of water its role is assumedby monazite. Titanite is the most abundant pressureequal to total pressure.Xenoliths in the central accessorymineral in the coarse-grainedgranitoids. It forms sub- granitoids were concluded to have equilibrated at 725 + hedral to euhedral, wedge-shapedgrains up to 3 mm in length. 102'C and the border granitoids aI 647 + 2C C on the Titanite has a small variation in composition with substitutions basis of Fe-Mg partitioning between garnet, biotite, and generallyless I Al and 7o/oFe3+ for Ti. F substitution of than 50/o cordierite. Although not quantified, /o, was concluded to in the O, site is lessthan l0o/o(App. 1). differ betweenthe central and marginal granitoids. Late minerals For the assemblagebiotite + magnetite + alkali feld- : feldspar + The late minerals in the Liberty Hill ganitoids include albite, spar, the equilibrium annite + VzO., alkali chlorite, titanite, epidote, muscovite, marcasite, hematite, cal- magnetite + HrO can be written. For this equilibrium cite, laumontite, prehnite, alkali feldspar, rutile, hematite, and Wones(1972) determined that logfn o:7409/T + 4.25 + - - hyalite. t/zLog f6, + 3 log Xr,*' * 2log Xot log 4.a15;r6, Iog Many of the biotites and amphibolesare intergrown with chlo- aF"3o4,where I is absolute temperature, X."*2 is the frac- rite + rutile, which are believed to be subsolidus replacement tion ofFe+2 in all the octahedral sites ofbiotite and thus = products. The chlorites have Fe/(Fe + Mg) 0.6 (App. l), sim- not Fe/(Fe + Mg), Xo" is the fraction of OH in biotite, ilar to the original biotites and amphiboles, and are ripidolites and a is the activity of the KAlSi3O, component in alkali and brunsvigites according to the terminology of Hey (1954). feldspar and Fe.Oo component in magnetite. This rela- Mn contents are less than 0.10 atoms per l8 anions. Fluorite provides a means to relate several physical and occursin lenticular massesin altered biotites, an occurrencethat tionship physical is believed to be secondarywith the F derived from the altera- compositional variables. Becausemany of the tion of the F-bearing biotite. Lenticules of prehnite or alkali parametersfor the Liberty Hill are unknown, the calcu- feldspar also occur in biotite. lations for the compositions of the Liberty Hill minerals Secondarytitanite foms anhedral, poikilitic grains rimming were perfbfined at a variety of temperatures,f.rn, and'fo, magnetite,ilmenite, and biotite. Its major-element composition (Fig. 7). Stability curves for the Liberty Hill biotites were overlaps that of primary titanite, but can differ in trace-element calculated for both absolute values of/o, and conditions (Speeret al., 1981). composition of appropriate solid f, bufers. The stability curves are Epidote occurs as skeletal intergrowths with biotite and am- based on the following assumptions: (1) the activity of phibole, as epitaxial rims on allanite, and as a saussuritization KAlSi,O* is 0.8. This is obtained from the activity-com- product in plagioclasecores. Microprobe analyses(App. 1), show positional model of Ghiorso (1984) using the that the epidotescomposing the saussuriteare more aluminous, relations with less than 16 molo/opistacite, than matrix , which estimatedoriginal magmatic feldsparcompositions in the contain between 25 and 34 molo/opistacite. Mn content is less Liberty Hill pluton. (2) The activity of FerOoin magnetite than 0.10 molo/opiemontite in the saussuriteepidotes and be- is 1.0.(3) The averagebiotite X."*2is 0.5 and the X"" is tweenO.22 and l.l3 molo/oin the matrix epidotes. 0.9. (4) The oxygen-fugacityvalues for the solid bufers Secondarywhite has two occurrences.The first is as dis- are from Chou (1978) for MH, from Huebner and Sato crete poikilitic or skeletal grains of white mica intergrown with (1980)for NNO, from Hewitt (1978)for QFM, and from biotite. The secondis as fine-grainedmasses in the coresof pla- Wones (1981)for TMQHIL. TMQHIL is the titanite * gioclases As noted above, these resulting from saussuritization. magnetite + qtartz + hedenbergite * ilmenite buffer are interpreted as being secondary and generally have givenby log.fo,: -30939/T + 14.98+ 0.142(P - l)/ the samecomposition as the primary muscovitesexcept for their Z, where 7is in kelvins and P is in bars. Additional limits lower Ti contents. : In weatheredgranite samples,supergene alteration ofthe sul- on the probable conditions were Pn"d P,.o,from Burn- fides is common. Pyrite and chalcopyrite have rims of hematite ham et al. (1969), the stability of muscovite + qoarlz and hydrous iron oxides. Some pyrite is altered to marcasite. from Chatterjeeand Johannes(1974), and the minimum- SPEER:EVOLUTION OF MAGMATIC AFM ASSEMBLAGES IN GRANITOID ROCKS 871 melting reaction in the system from Johannes (1984). Ptoto,= 4.5 kbor The stability calculationsfor the igneousbiotites of the Liberty Hill pluton and conditions derived from the con- tact aureole show that water and oxygen fugacities in- creasedfrom the central granitoids to the marginal gran- o itoids whereas temperature decreased.In Figure 7, the conditions of the marginal granitoids are constrained to o lie in the region below the curve Pn"ia: P,o,",,above the E granite solidus, and in the stability field of muscovite + qaartz. This small area, perhaps fortuitously, also in- cludes the temperature estimated for the border granit- oids. Estimated water fugacity would be 2.75 kbar, cor- respondingnearly to conditions of &",0 : P,.,, (Burnham et al., 1969),andforwould be approximatelyl0-15 8. The conditions for the central granitoids are less constrained and lie below Po,,o: P,o,or,above the granite solidus, and to the low-temperature side of the QFM solid buffer. If the estimated temperature of 725C and water pressure 500 600 700 800 900 of Po-u: 0.5P,o,",corresponding to.fnro: 1.7 kbar are temperoture,"C correctfor the xenoliths,thef, was about l0-'5, slightly Fig.l. Thebiotite + magnetite+ alkalifeldspar assemblage more oxidizing than the conditions of the TMQHIL and stabilityin the LibertyHill plutoncorresponding to the equilib- NNO solid buffers (Fig. 7). rium annite+ t/zOr: alkali feldspar+ magnetite* HrO over The X."*' and Xo" values of biotites from the amphi- a rangeof temperaturesand /rro for both absolutevalues of/o, bole * biotite granitoid core facies and the biotite gran- andconditions of the MH (: magnetite+ hematite),TMQHIL (: + + qrafiz + hedenbergite+ ilmenite), itoid rim facies are nearly identical. Originally, this sug- titanite magnetite NNO (: nickel + nickel oxide),and (: quartz + fayal- gested the possibility of subsolidus re-equilibration at QFM ite + magnetite)solid buffers.Other calculatedcurves in the pluton. uniform conditions throughout the However, these diagramare Po",o : P,.*,(Burnham et al., 1969),stability of mus- calculations of Liberty Hill biotite stability from the es- covite+ quartz(Chatterjee and Johannes, 1974), and beginning timated conditions of the different facies derived from of meltingin the granitesystem (Johannes, 1984). The temper- the xenoliths predict uniform magmatic biotite compo- atureand pressureestimates for the Liberty Hill granitoidsare sitions.It is a field demonstrationof the resultsof Wintsch from Speer(1981a). (1980) who found that the biotite composition in a bio- tite + K feldspar + magnetite assemblageis more or less indifferent to the fluid composition at unspecifiedP, Z, ro-edenite, ferro-edenite + biotite, biotite, muscovite + f^ro, a\d for. However, the interpretation that conditions biotite, and ferro-edenite - ferroan pargasite + biotite. in the Liberty Hill pluton's central and marginal granit- The clinopyroxene-bearingassemblage is restrictedto en- oids differed is plausiblebecause the intensive parameters claves or their immediate vicinity. Most of the pluton is have been determined from independent sources. dominated by the central, coarse-gtainedferro-edenite + The differencesin oxygenfugacity and temperaturebe- biotite granitoids that show abundant evidence for re- tween the central and marginal faciesought to be reflected placement of ferro-edeniteby biotite. The central granit- in the oxide mineral compositions.The particular oxygen oids grade imperceptibly into coarse-grained biotite, fugacitiesand temperaturesobtained for the central and muscovite + biotite, or ferro-edenite - ferroan pargas- marginal facieslie along the sameilmenite compositional ite + biotite granitoids at the pluton margins. These isopleth; however, the ulv6spinel component in the mag- coarse-grainedfacies are intruded by the later and finer- netites of the central facies should be about 20 molo/o grained biotite granitoids. The mineralogical evolution higher than the amount of ulvdspinel in the magnetites among thesevarietal assemblagesis discussedin this sec- from the marginal facies (Spencerand Lindsley, l98l). tron. The compositions of the ilmenites throughout the pluton Clinopyroxene - actinolite - ferro-edenite. The oc- are comparablein the amount of exsolvedhematite; how- cwrence of clinopryoxene in the Liberty Hill pluton could ever, the magnetitesof the border contain only representeither crystallization from the melt, xenocrysts, traces of exsolved Ti oxides whereas the magnetites of or minerals in equilibrium with the melt. Once the central granitoids contain up to 25 volo/oexsolved present, it reacted with the melt, first producing actino- ilmenite. lite, then ferro-edenite.The sharp, discontinuous nature of the zoning in the amphiboles may indicate either in- Evolution of the rnineral assemblages complete reaction becauseof slow diffusion, a miscibility There are five AFM varietal mineral assemblagesin gap, or a discontinuity in crystallization conditions. The the Liberty Hill pluton: clinopyroxene - actinolite - fer- compositional gap is similar to though wider than, that 872 SPEER:EVOLUTION OF MAGMATIC AFM ASSEMBLAGES IN GRANITOID ROCKS

R3-924 *-l4 brolile -\K3_ ilquro e. B1 F

TRANSITION TO THE MARGINAL , BIOTITE - I ,. MUSCOVITE+ BIOTITE GRANITOIDS

llquid \.<)DrollTe

omph i bole Liq =31 Liq+fl[1=31 CENTRAL FERRO-EDENITE+ BIOTITE GRANITOIDS

EARLY CRYSTALLIZATION REACTIONS

nblende Liq = Hbl+ Bf

clrnopyr0xene 500 600 700 800 Liq+6p*+Act+Hbl Temperoture,"C Liq = 1161

Fig. 8. Regionsof distinct AFM liquidus topologiesin P-Z spaceand pertinent schematicphase relationships among hornblende, biotite, cordierite, garnet, aluminum silicate, and muscovite in AFM (A : ALO3-KrO-Na'O-CaO, F : FeO, M : MgO) liquidus projectionsfrom quartz, alkali feldspar,plagioclase, and water for granitoid (Abbott and Clark, 1979; Abbott, 1981, 1985). The surrounding AFM diagramsillustrate the evolution of crystallization reactionsin the Liberty Hill pluton: (a) The Liq + Cpx : Act : Hbl reaction observed in the enclaves and enclave-bearinggranitoids. (b) The probable Liq : Hbl * Bt reaction in the SPEER:EVOLUTION OF MAGMATIC AFM ASSEMBLAGES IN GRANITOID ROCKS 873 found by Czamanskeand Wones (1973),Mason (1978), written as Liq I + Hbl : Bt + Liq 2. Becausethe gran- and Chivas (1981)for other igneousoccurrences. At the itoids were emplacedas mixtures of melt * crystals,Liq elevated pressures,temperatures, and Fe contents ofthe I and Liq 2 would be the interstitial melt. The finer- amphiboles in the Liberty Hill, the compositional gap grained biotite granitoids intruding the pluton are be- shouldbe smallor nonexistent.Oba (1980)found no mis- lieved to be derived from the interstitial liquids of the cibility gap betweentremolite and pargasiteat magmatic crystallizing coarse-grainedgranitoids and thus can be temperaturesabove I kbar and found the gap decreased used as estimatesof these Liquid compositions. A con- with increasingFe content at lower pressures(Oba, 1986). straint on Liq I and Liq 2 in the reaction should be the Basedon thermochemical data, Graham and Navrotsky compositional evolution of the Liberty Hill magma as (1986) have suggestedthat disequilibrium rather than a shown by "liquid" variation diagrams. On the basis of miscibility gap is the more likely interpretation for such the variation diagrams for the Liberty Hill pluton (Speer compositional gaps.The Si content of the actinolite being et al., unpub. ms.), Liq 2 should differ from Liq I by >7.6 atoms per 24 anions also is evidencethat it is not decreasedSi, Al, Ti, Fe, Mg, and Ca, yet have generally a magmaticphase; Chivas (1981) concluded that Si : 7.3 the sameK and Na contents.As shown below, this liquid is the limit of hypersolidus crystallization on the basis of variation could only be accomplished with the partici- Leake's(1971) slggestions about the compositionallim- pation of other phasesin the reaction. Indeed, the reac- its of igneousamphiboles. tion may be the reason that K and Na remain relatively In the absenceof a miscibility gap, the actinolite rims constant, an atypical trend in a crystallizing melt. on clinopyroxene must be incomplete reaction products The least-squares petrologic-mixing-model program of xenocryst + melt or restite + melt, and the actinolite cENMrx(Le Maitre, l98l) was used to balancethe an- did not crystallize from the Liberty Hill magma. If the hydrous componentsof the various Hbl + Liq : Bt mod- actinolite is not magmatic, it is unlikely that the clino- el reactions. A finer-grained granitoid (K3-72) was used pyroxene is either. However, clinopyroxene was present as an estimate of Liq I, and a more differentiated, finer- in the melt; forming inclusions in plagioclaseand reacting grained biotite granitoid (K3-924) was used as an esti- with the melt to produce a more magesianferro-edenite mate of the product, Liq 2. Mineral compositions from and giving amphibole-biotiteKD values between0.86 and K3-1303.8,in the samedrillhole as K3-72, were usedas 0.99 (Fig. 6a). The reaction sequenceclinopyroxene r estimatesof the phasesthat may have participated in the actinolite - ferro-edenitecould have occurred either be- reaction (Tables l, 2, and 3). The feldspars and quarlz fore or after biotite beganto crystallize. Becausethe bio- were assumedto have end-member and ideal composi- tite compositions appear unafected by the presenceof tions, respectively. All Fe was calculated as FeO, and the clinopyroxene, the Liq + Cpx (xenocryst, restite) - analyseswere recalculatedto l00o/oon an anhydrous ba- actinolite ' ferro-edenite sequencemust have occurred sis.Magnetite was assumedto be 93.090/oFeO. before the appearanceofbiotite, suggestingthat the early Model I (Table 4) is the essentialLiq I + Hbl : Liq crystallization reaction in the Liberty Hill magma was 2 + Bt reaction. The differencesindicate insufficient Si, Liq: Hbl (Fig. 8b). Ti, Fe, Mg, and Na and excessAl and Ca to balancethe Ferro-edenite + biotite. The ferro-edenite + biotite reaction. Ilmenite is ideal as a source for Ti. Anorthite granitoids are the dominant crystallization products of or titanite can serve as Ca sinks; Al, K, and Na sinks are the Liberty Hill magma. The widespreadtexture of bio- albite and K-feldspar. Si and Fe can be balanced by use tite replacingferro-edenite is evidencethat crystallization of magnetiteand quartz. Model 2 (Table 4) is the reaction conditions and compositions at the level of emplacement incorporating theseremaining minerals. The reaction can were those of the reaction Liq + Hbl : Bt describedby be acceptablybalanced, as would be expected,by adding Abbott (1981). relatively simple minerals and more phasesthan com- In its simplest form, the Liq + Hbl : Bt reaction would ponents. Excellencein balancing the reaction does not use the melt as both source and sink of all the necessary prove that it occurred but does show that biotite may be components to complete the reaction and thus is better produced from amphibole while allowing the melt to con-

ts magma. (c-'e)There are three possiblereasons for the Liq + Hbl : Bt reaction ofthe central, coarse-grainedferro-edenite + biotite granitoid: (c) the low-temperatureend of the Hbl-Bt liquidus boundary is odd, (d) the low-temperature end of the Hbl-Bt liquidus boundary is odd but with different geometry from (c) becausethe amphibole is pargasite,or (e) Liq has moved from the Hbl into the Bt field. (f+) With completion of the Liq + Hbl : Bt reaction by the consumption of Hbl in the marginal, coarse-grained granitoids, the crystallization reaction is (f) Liq : Bt or, more rarely, (g) Liq : Bt + Ms. (h) the finer-grainedbiotite granitoids are concludedto be interstital melt separatedfrom the coarse-grainedgranitoids in which only Bt crystallizesby the reaction Liq : Bt. The estimated conditions of the central granitoids are725"C, P.u E 4.5 kbar, Pu",o= 0.5P,"o,,andfo, = 10-t5 above to the NNO and TMQHIL solid buffers. The maryinal granitoids yield conditions of 647'C, Pnuia= Poar, and fo, = 10-16,relatively more oxidizing. 874 SPEER:EVOLUTION OF MAGMATIC AFM ASSEMBLAGES IN GRANITOID ROCKS

TneLe4. Coefficients(N, wt%) of least-squaresmixing models for progressivelymore inclusive magmatic Liquid 1 + Hbl : Bt + Uquid 2 and subsolidusHbl : Bt reactionsin the LibertyHill pluton,South Carolina

sio, Tio, Alro3 FeO MnO Mgo CaO Naro KrO Model 1 Reactants Liquid1t 99.95 72.52 0.35 14.74 1.95 0.04 0.41 1.55 3.08 5.37 Hbt- 0.05 44.51 1.51 9.02 21.99 0.88 8.48 10.69 1.76 1.17 Products Liquid2f 95.14 74.46 0.15 14.18 1.04 0.04 0.20 1.18 3.58 5.17 Bt- 4.86 37.83 3.00 14.51 24.33 0.61 9.49 0.17 o.17 9.90 Difference -0.17 0.06 0.54 -o.22 -0.03 -0.24 0.43 -0.34 -0.03 Residualsums of souares:0.7297 Model2 Reactants Liquid1t 96.53 72.52 0.35 14.74 1.95 0.04 0.41 1.55 3.08 5.37 Hbt- 3.47 44.5'l 1.51 9.02 21.99 0.88 8.48 10.69 1.76 1.17 llmt <0.00 0.08 47.64 0.08 46.17 5.26 0.31 0.23 0.12 0.10 Products Liquid2f 84.30 74.46 0.15 14.18 1.04 0.04 0.20 1.18 3.58 5.17 Bt- 5.43 37.83 3.00 14.51 24.33 0.61 9.49 0.17 0.17 9.90 Qtz 3.69 100.00 An 3.93 43.19 36.55 20.16 KfS 1.92 64.76 18.83 16.92 Ttn. o.25 31.18 36.83 1.83 2.00 0.31 o.23 27.38 o.14 0.09 Mag o.47 93.09 Difference 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Residualsums of squares:0.0001 Model3 Reactants Hbt- 49.52 44.51 1.51 9.O2 21.99 0.88 8.48 10.69 1.76 1.17 An 42.62 43.19 36.65 20.16 Mag 5.04 93.09 llm' 2.82 0.08 47.64 0.08 46.17 5.26 0.31 0.23 0.12 0.10 Products Bt. 5.05 37.83 3.00 14.51 24.33 0.61 9.49 o.17 0.17 9.90 cht- 26.13 29.23 0.16 20.87 34.57 1.01 13.77 0.18 0.11 0.10 Ep' 52.64 39.79 0.17 23.81 12.40 0.52 0.22 22.89 0.10 0.10 Ab 6.59 68.74 19.44 11.82 Qtz 3.69 100.00 Ttn- o.25 31.18 36.83 1.83 2.00 0.31 0.23 27.38 0.14 0.09 Cal 0.79 56.03 Difference 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Residualsums of squares:0.0000

t Compositionof a less-evolveddike of fine-grainedbiotite granitoid K3-72. + Compositionof a more evolved dike of fine-grainedbiotite granitoid K3-924. - Mineral analysisfrom the coarse{rained amphibole + biotite granitoid K3-1303.8(Tables 1J).

tinue its magmatic trend by involving other minerals in overall reaction, as would be expected.In order to better the reaction in amounts commensuratewith their modal examine the Liq + Hbl : Bt reaction, albite was not abundances.Biotite appears to be replacing a subequal included. amount ofhornblende. Textural evidencefor the reaction The averageKo value for the coexistingferro-edenite + is the replacement of ferro-edenite by biotite (Figs. 3a, biotite of the Liberty Hill is 1.04, with a range of 0.96- 3b) and rims of secondarytitanite on ilmenite. The pro- 1.17 (Fig. 6b). Although many coexisting metamorphic duction of anorthite component could show up in a more hornblendesand biotites have K" values near l.0, Mason calciczone in plagioclase.However, the calcic zone would (1985) has noted that in igneous rocks, hornblendes are be minor becauseof the small modal abundanceof the more magnesianthan the coexisting biotites (Kanisawa, products. Plagioclasesdo have oscillatory zoning, but it 1972).Mason (1985)found that, for the coexistinghorn- is not clear how a particular calcic zone can be correlated blendes and biotites in the Peruvian Coastal batholith, with the Liq + Hbl : Bt reaction. the Ko is about 0.63. The only exceptionsin the Peruvian Albite has been omitted from these model crystalliza- granitoids are samples in which biotite has formed by tion reactions.Addition of NaAlSi.O, as a phasechanges replacementof hornblende. In theseinstances, Ko values the reaction to the dominant ganitoid crystallization re- are near 1.0. and Mason concluded that the distribution action Liq : An * Ab + Kfs + Qtz, and the Liq + coefficient can be used to distinguish biotites that crys- Hbl: Bt reaction is subordinated to less than loloof the tallized directly from a melt from those formed by re- SPEER.:EVOLUTION OF MAGMATIC AFM ASSEMBLAGES IN GRANITOID ROCKS 875 placement of hornblende. Such a compositional distinc- Marginal facies. The possible origins of the three va- tion is consistentwith the apparent formation of biotites rietal AFM mineral assemblagesin the coarse-grained by the replacement of ferro-edenite in the Liberty Hill biotite, muscovite + biotite, and ferroan pargasite+ bio- pluton. The absenceof more "igneous" Ko values may tite granitoids along the margins of the Liberty Hill plu- reflect the difrculty of re-equilibrating the amphibole. ton have beenexamined previously by Wenner and Speer The Liq + Hbl : Bt reaction in the Liberty Hill pluton (1986). They concluded that the marginal facies compo- could have taken place as a result of either equilibrium sitionally overlap the central granitoids, which argues or disequilibrium crystallization. Abbott (1981) has con- against significant differentiation. The marginal faciesdo cluded that the low-temperature end of the Bt-Hbl liqui- show minor amounts of wall rock-magma interaction that dus boundary is odd; thus, the crystallization path could affectedsome trace-elementand oxygen-isotopecompo- encounter a higher-temperature even reaction, Liq : sitions. Hbl + Bt (Fig. 8B), after the supposedinitial Liq : Hbl The varietal biotite assemblagecould be produced eas- reaction (Fig. 8a). With continued equilibrium crystalli- ily from the central granitoids if Hbl is exhaustedbefore zalion, liquids following the boundary would crystallize Liq in the reaction Liq + Hbl : Bt. Estimations of con- by the odd reaction, Liq + Hbl : Bt, observedthrough- ditions for the marginal facies yield lower temperatures out the pluton. It is possible that the melt encountered and higher water pressuresthan those for the central the Hbl-Bt liquidus boundary where the equilibrium Liq + granitoids; the interpretation is that the marginal-facies Hbl + Bt is odd; this would eliminate a need for the conditions are conditions of final re-equilibration. Be- intervening even reaction. Abbott (1981) has drawn a causeit might be assumedthat at some earlier instance fairly gentle liquidus boundary; however, the composi- the conditions in the Liberty Hill magma were relatively tions of the Liberty Hill amphiboles and biotites require homogeneous,an extended reaction or re-equilibration a sharper changein direction in the AFM diagram (Fig. history for the marginal rocks is implied, allowing the 8c) for an odd reaction. Alternatively, the sharp change complete reaction of Hbl. Once the amphibole has dis- in liquidus direction could be in the larger compositional appeared,the reaction equation would be Liq : Bt (Fig. spacerather than in the simple AFM projection or could 8I). The extended reaction history of the maryinal rocks be made less sharp by interpreting the pargasiterims as may have beenfeasible because of a minor influx of water igneousminerals (Fig. 8d). from the wall rocks (Wenner and Speer,1986). The in- With disequilibrium crystallization, the interstitial liq- creasedarro could also shift the AFM liquidus bound- uid could move from a position in the Hbl field (Fig. 8a) aries if the Liq + Hbl : Bt reaction resulted from dis- to one within the Bt field (Fig. 8e). Such an abrupt move- equilibrium crystallization. ment could result from either a change in the positions The origin of the biotite + muscovite granitoids in the of the AFM field boundaries or a change in melt com- northern margin of the pluton is probably the extreme position becauseof crystallization or contamination. The consequenceof events that formed the marginal biotite movement of the AFM field boundarieswould be caused granitoids. Decreasingtemperature and increasingwater by a change in physical conditions, most likely P.*, or pressure would allow the appearanceof the muscovite Po",o.Increasing &,ia would increasethe size of the liqui- field in the AFM topology (Fig. 8g)and the crystallization dus field for biotite (Abbort, l98l). reactionLiq : Bt + Ms (Abbott, 1985).The absenceof It is possiblethat the reaction ofbiotite replacinghorn- intervening assemblagescontaining cordierite and alu- blende is a subsolidusone. Using what are interpreted as minum silicate more than likely results from their re- the subsolidus minerals intergrown with hornblende- stricted occurrencein P-T spaceor from subsequentre- chlorite, epidote, calcite, and quartz-and their compo- actlons. sitions from sampleK3-1303.8 (Table 3), a reaction to The occurrenceof amphiboles with aluminous ferroan produce biotite from amphibole without involvement of pargasiterims coexisting with aluminous biotites along a melt can be written (Model 3, Table 4). Although fea- the border of the pluton is not readily predicted. The sible, this reaction is consideredunlikely to have caused irregular zoning of the amphiboles from ferro-edenite to the replacementof amphibole by biotite becausethe pro- ferroan pargasiteis essentiallya manifestation of a rapid portions of the replacement products are not consistent increasein Al with only a small increasein Fe/(Fe + Mg) with their modal abundances.Biotite or chlorite, rather (Fig. 2b). The occurrenceas thin rims is evidence of a than epidote, are the most abundant replacementprod- late feature; however, the more aluminous and magne- ucts. Locally, biotite replacementofamphibole occurred sian nature ofthe coexistingbiotites (Figs. 4, 6c) indicate in the absenceofchlorite, epidote, albite, or calcite, and that the biotites equilibrated with the Al-rich amphibole the replacementof amphibole by chlorite, epidote, albite, nms. or calcite occurred in the absenceofbiotite. In addition, The occurrenceof aluminous amphiboles in metamor- late-stagesubsolidus or deuteric reactions in granitoids phic rocks has been attributed to extremely aluminous producemagnesian actinolite (Mason, 1978; Lanier et al., rock compositions, differing phaseassemblages, differing 1978;Chivas, 1981),not a ferro-edenite.For theserea- temperatures(Bunch and Okrusch,1973), increasedpres- sons,biotite replacementof amphibole in the Liberty Hill sure (Leake, 1965), and increasedoxygen fugacity (Mor- pluton is interpreted as a magmatic process. teani, 1978).The granitoids along the margin of the plu- 876 SPEER:EVOLUTION OF MAGMATIC AFM ASSEMBLAGES IN GRANITOID ROCKS ton have rock compositionsand silicateand oxide mineral rite + intermediate Cu-Fe solid solution + vapor assemblagescomparable to other granitoids in the pluton. (Vaughan and Craig, 1978).The high-temperatureinclu- Hammarstrom and Zen (1986)quantified the relation of sion assemblage,therefore, was pyrrhotite + pyrite + Al content of amphiboles in granitoid rocks with pres- intermediate Cu-Fe solid solution + magnetite, which sure. Using their geobarometer,the Al contents of the has a thermal stability at higher temperatures than the Liberty Hill pargasitescorrespond to estimated pressures granite solidus. The pyrite * chalcopyrite matrix assem- of 3.4 to 6.3 kbar, comparedto estimatedpressures of blage is the later sulfide assemblagethat crystallized in 3.1 to 4.7 kbar for the ferro-edenites.This simple expla- equilibrium with the magnetite + ilmenite * silicate as- nation is attractive, but there is no evidencethat the gran- semblage.This sequenceof sulfidesindicates that the ac- itoids in the northeasternsection ofthe pluton are deeper tivities of oxygen and sulfur appear to have increased or preserve a late crystallization at deeper levels. The during crystallization of the Liberty Hill magma. compositional trend of aluminous amphibole rims is op- posite to those found by Czamanskeand Wones (1973), CoNcr,usroNs Mason ( 1978),and Chivas( I 98I ), whoserocks have also Enclavesand a traversefrom core to rim ofthe Liberty recorded an increasein fo, and Si activity with crystalli- Hill pluton representa small portion of the cooling his- zation to form tremolite rims. The zoning to aluminous tory of the pluton and indicate the nature of the Liq + amphiboles with crystallization in the Liberty Hill pluton Hbl : Bt crystallization reaction and reactionsbefore and is also at odds with the expected compositional trend after it (Fig. 8). The prevalence of replacement textures from the tremolite + albite : edenite + quartz reaction and the rarity of Ko : 6Bfl/XB$/(W;/XF!) < 1 indicate qualitatively determined by Graham and Navrotsky that little biotite was presentbefore the Liq + Hbl : Bt (1986).Chivas (1981)has noted that amphibolescrystal- reaction. It is envisioned that the Liberty Hill magma lizing from magmasthat did not evolve a fluid (nonmin- initially begancrystallizing with the Liq : Hbl reaction, eralized trends) should have a limited compositional concurrentwith a Liq + Cpx - Act - Hbl reaction where range. In a magma that does evolve a fluid, magmatic the Cpx crystals were xenocrysts or restites (Fig. 8a). If amphiboleswould re-equilibrate or grow rims in the sub- the magma solidified as a result of equilibrium crystalli- solidus and have variable compositional trends depend- zation, the Hbl-Bt liquidus boundary would have been ing on the conditions and coexisting phaseassemblage. reachedwhere either (1) the Liq : Hbl + Bt even reaction The most reasonableconclusion is that the ferroan par- was encounteredbefore the Liq + Hbl : Bt odd reaction gasiterims are late magmatic or subsolidusfeatures. The (Fie. 8b) or (2) conditions of the Liq + Hbl : Bt odd averageK" value of 1.22,with a rangeof l.l3-1.30, for reaction were directly encountered (Figs. 8c, 8d). If the the coexisting ferroan pargasite + biotites may be indic- magma solidified as a result of disequilibrium conditions, ative of such nonmagmatic origin and explain Ko values the interstitial liquid would move from a position in the of > 1.0 found in other marginal Liberty Hill granitoids Hbl field (Fig. 8a) to one within the Bt field (Fig. 8e). not studied in suchdetail (Fig. 8c). The higher water pres- Such an abrupt changein the positions of the AFM field sures and oxygen fugacities of the marginal facies may boundaries, melt compositions, or both would result in favor such reactions and explain the restriction of such the Liq + Hbl : Bt reaction. Such changescould be ferroan pargasite + biotite assemblagesto the margin of brought about by movement of magma, adding or re- the pluton. moving melt, or an influx of water from the country rocks. Finer-grained biotite granitoids. The flner-grainedbio- Perhaps significantly, the finer-grained biotite granitoid tite granitoids are interpreted as having crystallized from magmasare believed to representthe interstitial melt that interstitial melts, extractedfrom the coarse-grainedgran- separatedfrom the coarse-grainedrocks at this stage.The itoids. The multiple compositions of these rocks (melts) interstitial melt of the coarse-grainedgranitoid was crys- indicates that they separatedeither at several different tallizirlg Bt by either Hbl + Liq : Bt or Liq : Bt and, times or from different volumes with differing degreesof upon separation,the Liq : Bt reaction would be expected crystallization. Interstitial melts of the coarse-grained to be the separatedmelt's crystallization reaction (Fig. granitoids were producing biotite by the Hbl + Liq I : 8h). Bt + Liq 2 andLiq: Bt crystallization reactions. If in- The Liq solidification of the central granitoids was terstitial melts separated to produce the finer-grained completed before the consumption of Hbl, preservingthe rocks, these crystallization reactions would explain the Hbl + Bt assemblage.Locally, the Hbl may have been essential similarity in compositions between biotites in sufrciently isolated from the Liq to prevent further re- the coarse-grainedrocks and biotites crystallizedfrom the action. The extended reaction history of the marginal extracted melts. coarse-grainedgranitoids allowed the Hbl to be exhaust- Pyrrhotite - pyrite. The inclusion assemblagepyrrho- ed before Liq in the reaction Liq + Hbl : Bt, whereupon tite + chalcopyrite + pyrite is the initial sulfide assem- the melt completed its solidification by the Liq : Bt re- blage that crystallized with an early magnetite + silicate action (Fig. 8f) and, locally, by the Liq : Bt + Ms reac- assemblage.However, this sulfide inclusion assemblage tion where there was sufrcient water pressure(Fig. 8g). is a re-equilibrated one. At temperatures between 400 Thus, difering coarse-grainedgranitoid mineral facies in and 600"C, pyrrhotite and chalcopyrite react to form py- the Liberty Hill pluton have not resulted from multiple SPEER:EVOLUTION OF MAGMATIC AFM ASSEMBLAGES IN GRANITOID ROCKS 877 intrusion or differentiation, but rather from extendedre- Huebner, J.S., and Sato, M (1970) The oxygenfugacity-temperature re- action histories of the marginal portions of the pluton. lationships of manganeseoxide and nickel oxide buffers. American Mineralogist, 55, 934-952 The extended reaction history may be causedby the in- Johannes,W (1984) Beginning of melting in the granite system Qz-Or- creasedavailability of water at the margins, possibly from Ab-An-HrO. Contributions to Mineralogy and Petrology,86,264-273 the country rocks. Kanisawa, S. (1972) Coexistingbiotites and homblendesfrom some gra- nitic rocks in southem Krtakami Mountains, Japan. Journal of the AcrNowlnncMENTS JapaneseAssociation of Mineralogists,Petrologists, and Economic Ge- ologists,67,332-344. This study is part of an investigation of low-temperature,geothermal Kretz, R. (1983) Symbols for rock-forming minerals. American Miner- energyresources supported by the U.S. Department of Energy (contract alogist,68,277-279. numbersEV-76-S-05-5103 and ET-78-C-05-5648to J. K. Cosrain,L. Lanier,G , Raab,W. J., Folsom,R.B., and Cone,S. (1978)Alteration of Glover III, and A. K. Sinha; and contract number DE-AC05-78ET27001 equigranular , Bingham mining district, Utah. Economic to J. K Costain and L. Glover III), with supplementalsupport from the Geology, 73, 1270-1286. U.S. GeologicalSurvey (contract l4-08-0001-G-685 to L. Glover III and Leake, B.E. (1965) The relationship betweencomposition of calciferous J. K. Costain). Assistancein this researchby S. W. Becker, S. S. Farrar, amphibole and $ade of metamorphism. In W.S. Pitcher and G W. and C. W. Russelland commentson the manuscript by J. Hogan, Richard Flinn, Eds., Controls of metamorphism. GeologicalJournal SpecialIs- N Abbott, and JamesF. Luhr are appreciated. sue l, Wiley, New York, 299-318. -(1971) On aluminous and edenitic homblendes.Mineralogrcal Magazrne,38,389-407. RrrnnnNcns -(1978) Nomenclature of amphiboles. Canadian Mineralogist, 16, Abbott, R.N, Jr (1981)AFM liquidusprojections for graniticmagmas, 50l -520. with special referenceto hornblende, biotite, and garnet. Canadian Le Maitre, R W (1981)oelnux-A generalizedpetrological mixing mod- Mineralogist,19, 103-l 10. el program.Computers & Geosciences,7,229-247 - (l 985) Muscovite-bearinggranites in the AFM liquidus projection. Mason, D.R. (1978)Compositional variations in ferromagnesianminerals Canadian Mineralogist, 23, 553-561. from porphyry copper+eneratingand barren intrusions ofthe Western Abbott, R.N., Jr., and Clarke, D.B. (1979) Hypothetical liquidus rela- Highlands, PapuaNew Guinea. Economic Geology, 73, 878-890. tionships in the subsystemAlrOr-FeO-MgO projected from quartz, al- Mason, G.H. (1985)The mineralogyand texturesof the Coastalbatholith, kali feldspar and plagioclasefor a (HrO) = l. Canadian Mineralogist, Peru. In W.S. Pitcher et al., Eds , Magmatism at a plate edge, the t7, 549-560. PeruvianAndes, p. 156-166.Wiley, New York. Bowen,N.L. (1928)The evolutionofthe igneousrocks. Princeton Uni- McSween, H.Y., Jr. (1972) Ar investigation of the Dutchmans Creek versity Press,Princeton, N.J., 334 p. , Fairfield County, South Carolina. South Carolina Division of Bunch, T.E., and Okrusch, M. (1973) Al-rich pargasite.American Min- Geology, S.C. StateDevelopment Board GeologicNotes, 16, 19-42. eralogist,58, 721-726. McSween,H.Y., Jr, and Nystrom, P.G., Jr. (1979)Mineralogy and pe- Burnham,C W , Holloway,J.R., and Davis,N.F. (1969)Thermodynamic trology of the Dutchmans Creek gabbroic intrusion, South Carolina. propertiesofwater to 1,000'C and 10,000bars. Geological Society of American Mineralogist, 64, 531-545. America SpecialPaper 132,96 p. Morteani, G. (1978) High-alumina amphiboles from the Penninic rocks Chatterjee,N.D., and Johannes,W. (1974)Thermal stability and standard of the westem Tauem Window (Tyrol, Austria). Neues Jahrbuch fiiLr thermodynamic properties of synthetic 2M,-muscovite, Mineralogie Abhandlungen, 133, 132-148. KAlr[AlSijOro(OH)r]. 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assemblagesand migration ofU in granitoid rocks. Economic Geology, rence of granites formed from primitive sourcerocks in the southem 76,2162-2175. Appalachian Piedmont. Geological Society ofAmerica Abstracts with Spencer,K.J., and Lindsley, D.H. ( I 98 I ) A solution model for coexisting Programs, 18, 785. iron- oxides. American Mineralogist, 66, I 189-l20L Wintsch, R.P. (1980) SupercriticalpE-pH diagrams,with applications to Stormer, J.C., Jr. (1983) The effectsofrecalculation on estimatesoftem- the stability ofbiotite. Contributions to Mineralogy and Petrology,73, perature and oxygen fugacity from analysis of multicomponent iron- 421-428. titanium oxides.American Mineralogist, 68, 586-594. Wones, D.R. (1972) Stability of biotite: A reply American Mineralogist, Thompson,J.B., Jr. (1981)An introductionto the mineralogyand pe- 57,316-317 trology of the biopyriboles. Mineralogical Societyof America Reviews -(1981) Mafic silicatesas indicators of intensive variables in gra- in Mineralogy,9A, 141-188. nitic magmas.Mining Geology,31,l9l-212. Vaughan,D J., and Craig,J.R. (1978)Mineral chemistry of metal sulfides, 493 p. Cambridge University Press,Cambridge, England. M,c.NuscnrprRECETVED Decrrn{srn 18, 1986 Wenner, D.8., and Speer,J A. (1986) Implications regardingthe occur- M.uuscnlpr AccEprEDINdev 26, 1987